Nanoscale BiFeO3 Research Articles

Fine Control of Multiferroic Features of Nanoscale BiFeO3 Powders Synthesized by Microwave-Assisted Solid-State Reaction

Fine Control of Multiferroic Features of Nanoscale BiFeO3 Powders Synthesized by Microwave-Assisted Solid-State Reaction

Pressure-induced phase transitions and structural evolution across the insulator–metal transition in bulk and nanoscale BiFeO3

The pressure-induced phase-transition sequences and structural evolution across the insulator–metal transition (IMT) in multiferroic BiFeO3 still remain unclear. Here we use a combination of high-pressure XRD, XAFS experiment and first principle calculation to investigate the pressure-derived structural transformations and structure-related properties in bulk and nanoscale BiFeO3 up to 55 GPa. A new Imma structure of BiFeO3 has been discovered in the pressure range of 48–52 GPa, which presents ferromagnetic (FM) metallic properties and therefore plays a key role in the IMT. Local structure study reveals that the Bi3+ cation gradually shifts toward the centrosymmetric position in BiO12 cluster during IMT. Besides, the detailed structural information of post-perovskite Cmcm phase has also been determined and thus the complete phase sequence up to 60 GPa is obtained. Our research provides a structural origin of the IMT and a new way to understand the FM release in BiFeO3 system.

Nonmonotonic particle-size-dependence of magnetoelectric coupling in strained nanosized particles of BiFeO3

Using high resolution powder x-ray and neutron diffraction experiments, we determined the off-centered displacement of the ions within a unit cell and magnetoelectric coupling in nanoscale BiFeO3 (≈20–200 nm). We found that both the off-centered displacement of the ions and magnetoelectric coupling exhibit nonmonotonic variation with particle size. They increase as the particle size reduces from bulk and reach maximum around 30 nm. With further decrease in particle size, they decrease precipitously. The magnetoelectric coupling is determined by the anomaly in off-centering of ions around the magnetic transition temperature (TN). The ions, in fact, exhibit large anomalous displacement around the TN which is analyzed using group theoretical approach. It underlies the nonmonotonic particle-size-dependence of off-centre displacement of ions and magnetoelectric coupling. The nonmonotonic variation of magnetoelectric coupling with particle size is further verified by direct electrical measurement of remanent ferroelectric hysteresis loops at room temperature under zero and ∼20 kOe magnetic field. Competition between enhanced lattice strain and compressive pressure appears to be causing the nonmonotonic particle-size-dependence of off-centre displacement while coupling between piezo and magnetostriction leads to nonmonotonicity in the variation of magnetoelectric coupling.

Magnetic transition at ∼150 K in nanoscale BiFeO3

From the high resolution powder neutron diffraction data, we obtain an evidence of spin-reorientation type transition around ∼150 K in nanoscale BiFeO3. The magnetic phase forms at ∼150 K, as a result of canting of the spins away from c-to a-axis by ∼6°, has been modeled by considering two antiferromagnetic phases of propagation vectors k = (0, 0, 0) and (0, 1/4, 1/4) and moments aligned along c- and a-axes. Direct magnetic measurement too, under both 0 and 50 kOe field, offers evidence for such a transition. While the experiment on a single crystal does not offer any evidence of magnetic transition at ∼150 K [M. Ramazanoglu et al. Phys. Rev. B 83 (2011), art. 174434(1–6)], clear observation of the transition in nanoscale BiFeO3 implies crucial role of the antiferrodistortive rotation of FeO6 octahedra, associated with large magnetization and enhanced canting angle, in inducing the transition. The transition is found to have a significant influence on the crystallographic parameters too, including the structural noncentrosymmetry which show distinct anomaly around 150 K.

Anomaly in structural noncentrosymmetry around TN n bulk and nanoscale BiFeO3

Using high resolution powder neutron diffraction data, we show that there is a distinct anomaly in the structural noncentrosymmetry around the magnetic transition point TN for bulk and nanoscale BiFeO3. It appears that the structural noncentrosymmetry - which gives rise to the ferroelectric polarization - is suppressed anomalously by ~1% (of the average noncentrosymmetry at above the magnetic transition) in the bulk sample and by ~12% in the nanoscale sample as the magnetic transition is approached from higher temperature. This observation shows that the multiferroic coupling improves in the nanoscale sample which is expected to brighten the application prospects of nanoscale BiFeO3 in nanospintronics-based sensor devices.PACS Nos. 75.70.Cn, 75.75.-c

A training effect on electrical properties in nanoscale BiFeO3

We report our observation of the training effect on dc electrical properties in a nanochain of BiFeO3 as a result of large scale migration of defects under the combined influence of electric field and Joule heating. We show that an optimum number of cycles of electric field within the range zero to ∼1.0 MV cm−1 across a temperature range 80–300 K helps in reaching the stable state via a glass-transition-like process in the defect structure. Further treatment does not give rise to any substantial modification. We conclude that such a training effect is ubiquitous in pristine nanowires or chains of oxides and needs to be addressed for applications in nanoelectronic devices.

Domain switching in spray pyrolysis-deposited nano-crystalline BiFeO3 films

Single-phase nano-scale BiFeO3 (BFO) films have been prepared under a controlled substrate temperature by a simple spray pyrolysis method. Fourier-transform infrared spectroscopy results indicate that single-phase BFO is deposited at low temperature. A magnetoelectric coupling to the anti-ferromagnetic and α–β phase transitions was observed at 350.2 and 832.8 °C, respectively. The capacitance–voltage (C–V) curves exhibit two coercive fields corresponding to ferroelastic (71° and 109°) and ferroelectric (180°) domains. Ferroelectric domain switching is dominant at lower electric fields. A non-volatile domain switching in the BFO films can prevent domain wall pinning and can enhance fatigue behavior in the films.

Ultrathin limit and dead-layer effects in local polarization switching of BiFeO3

Using piezoresponse force microscopy in an ultrahigh vacuum, polarization switching has been detected and quantified in epitaxial BiFeO3 films from 200 to about 4 unit cells thick. Local remnant piezoresponse was utilized to probe both ferroelectric properties and effects of imperfect electrical contacts. It was found that the shape of electromechanical hysteresis loops is strongly influenced by an extrinsic dielectric gap, primarily through the suppressing effect of the depolarizing field on the spontaneous polarization in the ultrathin films. Furthermore, statistical analysis of the hysteresis loops has revealed lateral variation of the extrinsic dielectric gap with sub–10-nm resolution. Robust and reproducible ferroelectric properties of nanoscale BiFeO3 indicate its potential for nanoscale applications in information storage and spintronics.

Multiferroic coupling in nanoscale BiFeO3

Using the results of x-ray and neutron diffraction experiments, we show that the ferroelectric polarization, in ∼22 nm particles of BiFeO3, exhibits a jump by ∼30% around the magnetic transition point TN (∼635 K) and a suppression by ∼7% under 5T magnetic field at room temperature (≪TN). These results confirm the presence of strong multiferroic coupling even in nanoscale BiFeO3 and thus could prove to be quite useful for applications based on nanosized devices of BiFeO3.

Particle size dependence of magnetization and noncentrosymmetry in nanoscale BiFeO3

The saturation magnetization (MS), antiferromagnetic transition point (TN), and the off-center displacements of Bi and Fe ions have been measured as a function of particle size in nanoscale BiFeO3. TN decreases down to ∼550 K for particles of size ∼5 nm from ∼653 K in bulk, while MS rises by more than an order of magnitude. Analysis of crystallographic structure from Rietveld refinement of x-ray diffraction patterns shows significant rise in off-center displacements of Bi (δBi) and Fe (δFe) ions within a unit cell with the decrease in particle size. The net unit-cell polarization PS too, is found to be larger in nanoscale regime.

Size-Dependent Infrared Phonon Modes and Ferroelectric Phase Transition in BiFeO3 Nanoparticles

One emergent property of ferroelectric nanoparticles is the sized-induced structural distortion to a high-symmetry paraelectric phase at small particle sizes. Finite length scale effects can thus be advantageously employed to elucidate ferroelectric transition mechanisms. In this work, we combine infrared spectroscopy with group theory and lattice dynamics calculations to reveal the displacive nature of the ferroelectric transition in BiFeO3, a room temperature multiferroic. Systematic intensity and frequency trends in selected vibrational modes show that the paraelectric phase is Pm3m and the lowest frequency A1 feature is the soft mode that drives the first order transition. Finite length scale effects are also evident in the electronic structure with a red-shifted band gap in nanoscale BiFeO3 compared with that of the rhombohedral film, a result that can impact the development of ferroelectric photovoltaics and oxide-based electronics. Taken together, these findings demonstrate the foundational importance of size effects for enhancing the rich functionality and broad utility of transition metal oxides.

Microwave synthesis and phase transitions in nanoscale BiFeO3

Nanosized multiferroic BiFeO3 powders were synthesized by a microwave combustion method. The average crystallite sizes of the samples stay at a same level with the ratio of fuel glycine 0.5 ≤ G/N ≤ 1.5 and it increases significantly with G/N = 2.0. An inhomogeneity of amorphous and microcrystallites is observed directly by HRTEM. A ferromagnetic hysteresis loop with a saturation magnetization (M S) of ~0.09 μ B /Fe has been observed at room temperature in the sample with a crystallite size of 53 nm, whereas other powders with much smaller crystallite size (~20 nm) will not saturate even at 20 kOe. These magnetic behaviors were ascribed to a combination of the magnetic enhancement effect of a decreased crystallite size and superparamagnetic mechanism.

Photocatalytic and magnetic behaviors observed in nanostructured BiFeO3 particles

Nanostructured BiFeO3 particles have been synthesized by a hydrothermal method, and the effects of particle size on photocatalytic activity and magnetic property of BiFeO3 were investigated. The optical absorption spectra indicate that the band-gap energy increases with decreasing crystalline size due to the quantum-size effect. The enhancement of room-temperature weak ferromagnetism can be observed in nanoscale BiFeO3 particles, which should be attributed to the size-confinement effect on the magnetic ordering. In addition, BiFeO3 nanoparticles with diameter about 5 nm show good photocatalytic performance by photodegradation of Congo red under visible-light (λ>400 nm) irradiation.

Ferromagnetism in nanoscale BiFeO3

A remarkably high saturation magnetization of ∼0.4μB∕Fe along with room temperature ferromagnetic hysteresis loop has been observed in nanoscale (4–40nm) multiferroic BiFeO3 which in bulk form exhibits weak magnetization (∼0.02μB∕Fe) and an antiferromagnetic order. The magnetic hysteresis loops exhibit exchange bias and vertical asymmetry which could be because of spin pinning at the boundaries between ferromagnetic and antiferromagnetic domains. Interestingly, both the calorimetric and dielectric permittivity data in nanoscale BiFeO3 exhibit characteristic features at the magnetic transition point. These features establish the formation of a true ferromagnetic-ferroelectric system with a coupling between the respective order parameters in nanoscale BiFeO3.

Particle size dependence of magnetization and phase transition near TN in multiferroic BiFeO3

We report results of a comprehensive study of the phase transition at TN (∼643K) as a function of particle size in multiferroic BiFeO3 system. We employed electrical, thermal, and temperature dependent x-ray diffraction studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime of 2–300K under a magnetic field of 100–10000Oe both on bulk and nanocrystalline systems. While in the bulk system a sharp endothermic peak at TN together with a broad feature, ranging over nearly ∼100K (ΔT), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at TN, is found to occur both at TO and TN across ΔT in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metastability increases in the nanoscale BiFeO3 with divergence between zero-field cooled and field cooled magnetizations below ∼100K and faster magnetic relaxation. Interestingly, in nanoscale BiFeO3 one also observes finite coercivity at lower temperature, which points out that suitable design of particle size and shape may induce ferromagnetism. The inhomogeneous distribution of Bi∕Fe ions and/or oxygen nonstoichiometry seems to be giving rise to broad features in thermal, magnetic as well as electrical responses.